Scientists baffled by 'black blob' threat to Florida sea

Marine biologists in America are struggling to understand the cause of a huge area of ocean off Florida's southern tip mysteriously turning black, which has chased away fish and tourists, and has possibly extinguished life on the seabed.

Marine biologists in America are struggling to understand the cause of a huge area of ocean off Florida's southern tip mysteriously turning black, which has chased away fish and tourists, and has possibly extinguished life on the seabed.

At its peak in February, the threatening cloud covered an expanse of 1,800 sq km (700 sq miles) in Florida Bay just north of the Florida keys, where the waters are usually an exquisite blue-green. It has been clearly visible from the air and from satellite photographs where it has had the appearance of a large black blob.

Fishermen complained in January that wherever the dark waters were, fish were absent. Environmentalists have reported sponges dying on the ocean floor and have suggested the blob is a warning sign of the damage being wreaked by humans on the ocean.

John Hunt, of the Florida Marine Research Institute in St Petersburg, said: "This is truly a detective story. We had events occur. We're trying to collect information today and project back to see what has caused this event." In recent weeks, the cloud has started to contract and drift to the south.

Most scientists believe that the culprit is some kind of unusual algal bloom. Any dense area of algae in the water would absorb sunlight and hence appear to turn the ocean black. Divers who have attempted to take samples of the affected water have spoken instead of a "creepy green".

Allison McDonald, also from the research institute, conceded: "We don't know what it is. It appears marine life is avoiding it, which is another mystery. How come they know to avoid it?"

One reason may be that such a concentration of algae is lowering the levels of oxygen in the water. Fish need oxygen and would therefore flee areas without it. The divers sent down to investigate have also reporting seeing small numbers of small fish dead on the seabed. So far, officials have seen no reason to declare the blob a threat to human health.

In past years, the Gulf Coast of Florida has been struck by so-called red tide algal blooms that have killed large numbers of fish. The tides, which were particularly severe in the mid-1990s, also stung the eyes of humans and irritated those with respiratory problems.

The researchers admit that they may never get the chance fully to understand where the black blob came from or how it formed. No samples were taken when it was at its most intense and now it has become almost too dispersed and diluted to yield meaningful data.